Electrodes of battery cells often comprise an expanded metal sheet, the openings and surfaces of which carry a chemically active powder. A method and apparatus for producing an expanded grid is disclosed in U.S. Pat. No. 3,760,470. U.S. Pat. No. 4,291,443 discloses a conventional three roll cluster (where the strip passes between first and second rolls in a preforming/slitting stage, and then between the second and a third roll in a final slitting stage) for preforming and slitting a sheet to produce an expanded metal mesh, the entirety of which is incorporated herein by reference. U.S. Pat. No. 4,297,866 discloses an improved process for producing expanded metal sheet by concurrently slitting and preforming the strip. In a preferred arrangement in the '866 patent, the length of one side of a triangle corresponding to the tooth configuration, and collinear with the leading tooth surface, is less than the length of another side of the triangle corresponding to and collinear with the trailing tooth surface. In this same triangle, the entry angle formed between the side of the triangle corresponding to the leading surface and the base of the triangle is less than 90.degree.. In other words, the convexly shaped tooth surfaces used to deform slit segments out of the plane of the sheet or strip are asymmetrically shaped. Problems have been experienced with asymmetrical cutters, however. For example, when mesh is required to be flattened to within 0.015-0.012" of the strip thickness, and the top wires are very heavy (0.050-0.060") as required for positive grids with high conductivity, the "short" leg of the tooth segment or surface develops a kink when flattened. This kink prevents the mesh from being uniformly thin and often causes jams, resulting in stoppages in the paster apparatus. Precision pasting to the required weight thus becomes problematic.
I have now discovered that forming the cutter tooth with a symmetrical shape makes the mesh easier to flatten. One of the problems, however, with choosing the correct angle for the cutter "triangle" relates to the permissible elongation of the lead strip. Experience has shown that 45.degree. is the maximum entry angle. When this is translated into a rotary process, allowance must be made for the fact that the tool rotates and the leading edge is reduced, and the trailing angle (as defined between the base of the triangle and the trailing surface of the tooth) is increased by the angle between vertical and when the tool first enters the lead strip. If the trailing angle becomes too large, excessive elongation will result. The '866 patent purports to make this allowance by utilizing asymmetrical surfaces, but this results in the problem mentioned above.
In accordance with this invention, a two or three roll cluster is utilized, with the geometry of the cutter teeth for the roll pair which performs the strip based on calculations when the tool exits the strip. For purposes of this invention, in terms of its incorporation in a three roll cluster, the focus is on the interaction of the first and second rolls, i.e., during the preforming/slitting stage, while the interaction between the second and third rolls in the final slitting stage forms no part of this invention. The trailing edge angle of the tooth is always less than 45.degree., and is determined as a function of the radius of the tooth root, the depth of the tooth and the thickness of the strip. The entry angle is then made equal to the trailing angle, and therefore it follows that the nose angle (the angle between the leading and trailing surfaces, at the apex of the tooth) is always greater than 90.degree..
In its broader aspects, therefore, this invention relates to an improvement in a method of forming expanded metal sheet from a deformable strip comprising preforming and slitting the strip by intermeshing tooth segments of at least an upper disk and a lower disk with the strip passing therebetween, wherein the tooth segments each include linear leading and trailing surfaces joined at an apex defining a nose angle, said leading and trailing surfaces intersecting respective tangents to a radius of the roll at circumferentially spaced locations defining entry and exit angles, respectively, and wherein the exit angle is selected in accordance with the formula: ##EQU2## where R=radius of tooth root;
D=depth of tooth: and PA1 T=strip thickness. PA1 D=depth of tooth; and PA1 T=strip thickness..
In a related aspect, the invention relates to an improvement in a method of forming expanded metal sheet from a deformable strip comprising preforming and slitting the strip by intermeshing tooth segments of at least an upper disk and a lower disk with the strip passing therebetween, wherein the tooth segments each include linear leading and trailing surfaces joined at an apex defining a nose angle, said leading and trailing surfaces intersecting respective tangents to a radius of the disk at circumferentially spaced locations defining entry and exit angles, respectively, and wherein the entry angle is selected in accordance with the formula: ##EQU3## where R=radius of tooth root;
The above described tool and related method produce a uniformly thin, expanded metal product for use as a battery grid that may be accurately pasted to the required weight.
Additional objects and advantages of the subject invention will become apparent from the detailed description which follows.